They are juggling a few different ideas. The first of which is rotational spawn points. In other words, a power weapon does not always spawn in the same place, but cycles through two (or more) different locations. The second step is setting a limit on how many power weapons are available on a map at any given time. Instead of only allowing one sniper to be held, the limit could be upped to four. By combining higher limits with rotating spawn points, it will be very difficult (hopefully impossible) for a team to hold all of the power weapons at once.
Another new addition that curbs the desire to camp is grenade limits. Players are now limited to two of every grenade type. There are now three grenade types, so you can carry a total of six grenades at any time. The idea behind this is that players will have to come out of hiding to restock on grenades.
What about duel wielding? Dual wielding was never intended to be the defining feature of Halo 2, but it quickly became overused, primarily thanks to the underpowered starting weapon, the SMG. The SMG was weak enough that it was quickly overpowered by any other weapon. This inspired people to find a dual-wield weapon as quickly as possible, or risk death. Thus, Halo 2 became a race for all the wrong weapons. To combat this, Bungie set out to create the perfect starting weapon. As Bungie put it in the latest copy of EGM, "[the starting weapon] shouldn't do head shots, because then the game is just about head shots. And it shouldn't be dual-wieldable, because then the game becomes about dual-wielding." The other important thing is that the starting weapon should hold its own in a firefight. Whatever you spawn with should be able to take out any other weapon (if you're good enough). Enter the new MA5C assault rifle. Great at close to mid-range fighting, it's Bungie's hope that this is the perfect starting weapon. Also of note, tracking has been greatly reduced on the plasma pistol, making the "noob combo" much less desirable.
The final change is something many may not have thought of before. Halo 2 employed the use of symbols to designate your team members. This works great if you want to note the position of your teammates, but it's lousy if you want to give orders. Many of the symbols aren't exactly easy to fit into important messages. Example, "Hey, guy with the yellow Triforce symbol, you're about to get sniped!" To keep communications simple, Bungie is implementing a new call sign system. Players will choose a three character designation -- one letter and two numbers -- like M47. It may sound strange, but in practice it's much easier to use call signs than it is to verbalize symbols (or Gamertags for that matter).
No doubt, these are only some of the things Bungie is doing to even out Halo's multiplayer experience. What other changes would you like to see?